The Tempest was the third and last of the fighters designed by Sydney Camm to enter operational service in World War II and, while perhaps not as well known as the preceding Hurricane and Typhoon, was nonetheless an excellent warplane that proved highly adaptable in terms of its production in variants with air-cooled and liquid-cooled engines. The Tempest was also one of the machines that characterised the very apogee of the piston-engined fighter design, and was one of the very few warplanes to have been both designed and placed in service during the course of the war.
Work on the concept that would mature as the Tempest began in March 1940 as part of the Hawker design office’s general studies to improve on the basic design of the Typhoon, whose prototype had started its flight trials in the previous month. One aspect of the core concept that secured initial attention was whether or not it was desirable to introduce a wing of reduced thickness/chord ratio and reduced area. As the initial results of the Typhoon’s flight test programme were assessed, the design team decided that a small and thinner wing was absolutely essential to the success of the new fighter as a high-performance type. The wing of the Typhoon had a thickness/chord ratio of 18% at the root, largely as a result of the need for considerable fuel capacity and the provision of heavy armament with a substantial ammunition load, and it was immediately clear that this was the primary reason for the Typhoon’s indifferent performance at medium and high altitudes, as well as a tendency toward buffeting and aileron reversal in high-speed dives.
The design team accordingly opted for a much slimmer wing, and in due course combined this with the result of an analysis that suggested the advantages of engine and oil coolant radiators located in the wing roots (and using air drawn through the leading edge) rather than in the type of large chin installation that constituted one of the most distinctive features of the Typhoon’s appearance.
Theoretical work on the aerodynamics of the desired wing continued during 1940, but little practical design work was possible because of the overridingly high priority accorded in May 1940 to the production and further development of the Hurricane in accordance with the orders issued by Lord Beaverbrook, the Minister of Aircraft Production, that design and production effort should be concentrated on only five British warplanes (the other four being the Armstrong Whitworth Whitley, Bristol Blenheim, Supermarine Spitfire and Vickers Wellington) at this moment of crisis following the Germans forces’ invasions of the Netherlands, Belgium and France on 10 May. This decision slowed the development of a number of important but forthcoming rather than immediately valuable warplanes, including the Typhoon, but full-scale work on more advanced designs was resumed after the crisis had passed with the British victory in the Battle of Britain. By the late summer of 1941, therefore, the design of leading-edge radiators for the original Typhoon wing had been completed with the project office designation P.1012 for what was at one time projected as the Typhoon Mk II.
It was then appreciated that a radiator installation buried in the original type of thick wing went only part of the way to overcoming the intrinsic problem of a wing of this type. After discussions between Camm and the Directorate of Technical Development at the Ministry of Aircraft Production, Hawker was authorised in March 1941 to embark on the full-scale development of a more advanced thin wing for the Typhoon Mk II. Specification F.10/41 was drawn for the proposed new fighter, which continued to be known in the project office as the P.1012, and detailed design work on the new wing began in September 1941.
The Hurricane and Typhoon had each employed a wing based on an American aerofoil created by the NACA (National Advisory Committee for Aeronautics), but for the new wing Hawker designed its own aerofoil with its point of maximum thickness moved considerably to the rear (37.5% chord). The root thickness of the new wing was 5 in (0.127 m) less than that of the Typhoon, and as a result of the need to carry the planned armament of four 20 mm Hispano cannon in the leading edges outboard of the disc swept by the propeller, the wing was given considerable chord, which resulted in a semi-elliptical planform with a thickness/chord ratio that decreased from 14.5% at the root to 10% at the tips. These last were originally quite pointed, resulting in a span of 43 ft 0 in (13.11 m), but it was later seen that little area would be lost but the rate of roll considerably enhanced by the adoption of cut-off tips that trimmed the span to 41 ft 0 in (12.50 m). Calculation also revealed that the buffet threshold for the new wing would be Mach 0.73 compared with the Typhoon’s figure of 0.64.
The main disadvantage of the thin wing was, of course, its reduced volume, and this translated into the limitation of the wing’s fuel tankage to a single small tank in the inboard section behind each wheel well. The need to provide adequate range therefore required that volume had to be provided in the fuselage for an additional tank, and the volume demanded near the centre of gravity position could be provided only by a lengthening of the fuselage: thus a new bay was inserted between the engine and the cockpit, increasing the length forward of the centre of gravity by 1 ft 9 in (0.53 m), and this had in turn to be balanced by an enlargement of the tail unit’s horizontal and vertical areas, although the new type was originally flown with the Typhoon’s standard tail unit.
A useful by-product of the removal of much of the fuel tankage from the wing to the fuselage was the provision of volume for the installation of the wing-root radiators, and it was with such a coolant installation that the type was schemed at the time that Hawker received the November 1941 order for two F.10/41 prototypes each with a powerplant of one Napier Sabre liquid-cooled H-type engine that was to be known in service as the Sabre IV.
The new fighter was clearly a development of the Typhoon, but as a succession of alterations were worked into the design, covering the full range of internal details, structure and external mould line, it became sensible to introduce a new name, and in January 1942 the designation Tempest Mk I was adopted for the F.10/41 fighter with the Sabre IV engine and leading-edge radiator installation, although it should be noted that Hawker was by this time seriously considering other powerplants as a result, largely, of the cancellation of the production programme for the Tornado, which was in essence a counterpart of the Typhoon with the powerplant of one Rolls-Royce Vulture liquid-cooled X-type engine.
At the time of the Tornado programme’s cancellation, Hawker had been contracted for additional Tornado prototypes with the alternative powerplant of one Bristol Centaurus air-cooled 18-cylinder two-row radial engine, with less advanced studies already under way for possible use of other engines, including the Wright R-3350 Duplex Cyclone air-cooled 18-cylinder two-row radial unit and the Fairey P.24 Monarch liquid-cooled H-24 engine comprising two vertically opposed 12-cylinder units. Only one Centaurus-engined prototype was built in addition to three Sabre-engined Tornado aircraft in the form of two prototype and one production machines.
The sole production aeroplane made its maiden flight on 23 October 1941, just before the placement of the Tempest Mk I prototype contract, and it was now decided that five more airframes should be included in the F.10/41 development programme. Three of these five prototypes were to have the powerplant of one Centaurus IV radial engine, but one of them (originally known somewhat prosaically as the Typhoon Mk II with Centaurus) was deleted before the name Tempest was adopted, thus leaving two prototypes to undergo development as Tempest Mk II machines. The other two prototypes were intended to have the Rolls-Royce Griffon liquid-cooled V-12 piston engine in the form of either the Griffon IIB, in which guise they would be Tempest Mk III machines, or the Griffon 61 in a ‘power egg’ installation to become Tempest Mk IV machines. Neither of these prototypes was built as a Tempest, although one did later fly as part of the development effort for Hawker’s next-generation fighter, the Fury designed to meet Specification F.2/43.
In structural terms the Tempest followed the Hawker concept, updated in a number of respects, that had been proved in the Hurricane and Typhoon. The Tempest was therefore a cantilever low-wing monoplane of light alloy stressed-skin construction, and its core was the oval-section fuselage, which included a semi-monocoque rear section: from front to rear, this fuselage carried the powerplant (supported by bearers extending forward from the firewall that was the forward end of the fuselage structure proper), the oil and main fuel tanks, the cockpit with the pilot accommodated under a fixed canopy based on that of the Typhoon and therefore accessed by a car-type side door, and the tail unit. This last comprised single horizontal and vertical surfaces skinned with light alloy except on the rudder, which was covered with fabric. The horizontal surface included a tailplane and elevator halves each incorporating an inset trim tab controlled from the cockpit, while the vertical surface included a fin and a balanced rudder that also incorporated a trim tab controlled from the cockpit.
The low-set wing was a two-spar structure with ribs and light alloy skins stiffened by spanwise stringers, and the main and rear spars constituted the front and rear members of the main structural torsion box, aft of which there was no primary structure in the area of the gun bays, allowing the guns to be installed and rearmed from a point behind the rear spar. As noted above, the wing was of semi-elliptical planform and this tapered in thickness and chord to its tips which were only slightly rounded. The centre section was flat, largely to reduce the length and therefore the weight of the main landing gear units, and its halves extended from the lower fuselage to a heavy outboard rib that supported the main landing gear units as well as the dihedralled outer panels. Virtually the full span of the wing’s trailing edge was occupied by outboard ailerons (each incorporating an inset trim tab that could be altered only on the ground) and inboard four-section split flaps that were operated hydraulically. The airframe was completed by the landing gear, which was of the tailwheel type with a single wheel on each unit: all three units were retractable by a hydraulic system, and while the tailwheel unit retracted forward into the underside of the rear fuselage, the main wheel units retracted inward into wells in the underside of the centre section, where they were fully enclosed by the fairing attached to each leg and two other fairings hinged to the inboard ends of the wells.
As noted above, the powerplant was based on one Sabre H-type engine, and this was installed in the front of the fuselage inside a light alloy cowling, drove a four-blade propeller of the constant-speed type fitted with a neat spinner, and discharged its spent gases via a row of ejector exhaust stubs on each side.
As work on the two Tempest prototypes continued during 1942, the development of the 2,500 hp (1864 kW) Sabre IV engine and its associated buried radiator installation was beset by problems. In these circumstances it was thought sensible to complete the first prototype with the powerplant of the Typhoon, namely the Sabre II engine rated at 2,180 hp (1625.5 kW), driving a four-blade de Havilland propeller of the constant-speed type with a diameter of 14 ft 0 in (4.27 m) and cooled by an undernose radiator installation. This expedient was adopted so that the airframe could be used for trials as soon as possible, and this machine recorded the type’s maiden flight on 2 September 1942 with the designation Tempest Mk V and, for the first two months of its trials, the original type of Typhoon tail unit. The second prototype first flew on 24 February 1943 with the nicely streamlined Sabre IV installation including the wing leading-edge radiator arrangement, and was otherwise similar to the first prototype including the early type of cockpit enclosure and the Typhoon tail unit.
Early trials confirmed that the Tempest had considerably better performance and handling than the Typhoon and, as rightly predicted, that the second prototype offered better performance than the first as a result of its greater power and the lower drag of its cleaner powerplant installation. The better performance included a maximum speed of 405 kt (466 mph; 750 km/h) at 24,500 ft (7470 m). Taking off in normally laden condition at a weight of 11,300 lb (5126 kg), the Tempest Mk I had a range of 669 nm (770 miles; 1239 km), climb to 15,000 ft (4570 m) in 4 minutes 15 seconds, and service ceiling of 39,000 ft (11885 m).
This was all very impressive, but the fly in the ointment for the Tempest programme was the fact that the Sabre IV was still beset by major problems, and this jeopardised the batch of 400 Tempest Mk I fighters that had been ordered in August 1942, one month before the first prototype’s maiden flight. The Typhoon was proving inadequate in the medium- to high-altitude interceptor role for which it had been designed, and only just starting to be seen as the highly capable low-level fighter-bomber for the ground-attack task that was to become its forte, and as a result the Royal Air Force had an urgent need for a fighter offering a genuinely effective medium- to high-altitude interceptor capability. It was therefore decided to forget the Sabre IV and its leading-edge radiator installation from short-term production plans and instead to start production with the Tempest Mk V with the Sabre IIA powerplant and radiator installation of the Typhoon.
This decision made it possible for Hawker to start manufacture of the Tempest at its Langley plant, and the first Tempest Mk V from this production line made its maiden flight on 21 June 1943. Since the inboard ends of the wing no longer accommodated radiators, it became feasible to install an additional fuel tank in the leading edges of the port and starboard halves of the centre section (such a tank was in fact generally installed only in the port half of the centre section), and the production standard further differed from that of the prototypes in having a revised cockpit enclosure (fixed windscreen and rearward-sliding canopy section of the clear-view type) and the larger tail unit introduced during flight testing of the prototypes.
The first 100 aircraft were completed to the Tempest Mk V Series 1 standard with a fixed forward-firing armament of four 20 mm Hispano II cannon with 200 rounds per gun and their muzzles projecting some 8 in (0.203 m) ahead of the leading edge, but all later aircraft were finished to the Tempest Mk V Series 2 standard with 20 mm Hispano V cannon whose shorter barrels did not protrude forward of the leading edge. There were also a number of minor structural modifications in the wing, spring tabs were fitted in the ailerons to replace the ground-set tabs, and the standard powerplant was the Sabre II in three sub-variants that changed from the Sabre IIA rated at 2,180 hp (1625 kW) in the first aircraft, via the Sabre IIB rated at 2,200 hp (1640 kW) in later aircraft and finally to the Sabre IIC rated at 2,260 hp (1685 kW) in the last aircraft.
The original contract for 400 aircraft (ordered as Tempest Mk I machines but then changed to Tempest Mk V fighters) had included 300 Typhoon airframes ordered from the Gloster company but manufactured by Hawker as Tempest aircraft. By the end of World War II Hawker had been contracted for an additional 749 Tempest fighters, but at this time 207 of these were cancelled and another 142 completed to Tempest Mk VI standard, resulting on a Tempest Mk V production total of 800.